1. Field of the Invention
The present invention relates to an optical filter and a tunable filter which are able to convert a spectrum of an optical signal into a desired spectrum.
2. Discussion of the Related Art
An optical filter and a tunable filter are widely used in many fields such as the optical communication and the spectroscopic analysis. In the optical communication field, optical filters each realizing various spectral shapes such as a wavelength selection function for selecting (passing or blocking) a certain wavelength, a gain equivalence function for equalizing spectra which passed through an optical amplifier to have gain different in each wavelength and for conforming their optical power levels, and an interleaving function for branching a WDM signal into signal groups of odd channels and even channels. As shown in IEEE Photon. Technol. Lett., vol. 19, no. 19, pp. 1442-1444, October 2007, a method for realizing speeding up an optical transmission signal by passing the optical transmission signal through an optical filter realizing complex spectral shapes has been proposed recently. For this reason, the spectral shapes required for the optical filter are increasingly diversified and complicated.
As a conventional optical filter and a tunable optical filter, a dielectric multilayer filter made by in turn stacking a plurality of materials with different refractive indexes, an optical filter employing an optical fiber Bragg grating made by applying modulation of a refractive index (a diffraction grating) on a core of optical fiber, and further a Fabry-Perot etalon filter using a multiple interference between two reflectors are used. The dielectric multilayer filter has great freedom of designing the spectral shapes, and thus is most widely used especially for realizing an optical filter having a specific spectrum.